1. Field of the Invention
The present invention relates to a multilayer chip capacitor. More particularly, the present invention relates to a highly-reliable embedded multilayer chip capacitor, and a printed circuit board having the same.
2. Description of the Related Art
With higher density and integration of electronic circuits, printed circuit boards lack a space for mounting a passive device. To overcome this, efforts have been under way to implement embedded devices or parts embedded in the board. Especially, various suggestions have been made regarding a method for embedding a capacitor, used as capacitive parts, in the board. In a method to embed the capacitor in the board, the board itself is utilized as dielectric material for the capacitor, and copper wiring is used as an electrode therefore. In another method, polymer sheets with high dielectric constant or thin dielectric films are formed inside a board to provide an embedded capacitor. Other methods include embedding a multilayer chip capacitor in a board.
Generally, a multilayer chip capacitor includes a plurality of dielectric layers made of ceramic material, and internal electrodes interleaved between the dielectric layers. The multilayer chip capacitor arranged inside the board can have high capacitance. For example, Japanese Laid Open Patent Application No. 2002-100875 discloses a technology for embedding a two-terminal multilayer chip capacitor in a core substrate. The embedded multilayer chip capacitor disclosed in the aforesaid document includes 2 external electrodes formed on both sides of the capacitor. According to the conventional technology, typically a thin-film multilayer chip capacitor is used to embed the capacitor in the board.
FIGS. 1(a) and 1(b) show a conventional two-terminal multilayer chip capacitor used as an embedded capacitor. Referring to FIGS. 1(a) and 1(b), the embedded chip capacitor 10 includes 2 external electrodes 11, 13 formed on both sides of the capacitor and a capacitor body 15 made of dielectrics. As shown in FIG. 1(b), first internal electrodes 23 and second internal electrodes 23 are arranged on opposed faces inside the capacitor body 15. First internal electrodes 21 are connected to first external electrode 11 and second internal electrodes 23 are connected to second external electrode 13, thus exhibiting opposite polarity.
To be embedded in the board, the multilayer chip capacitor 10 should be made thin. The thin multilayer chip capacitor 10 may tend to suffer from chipping 5 or cracking 6 in handling during or after fabrication thereof. The chipping or cracking undermines reliability of the capacitor and possibly causes malfunction thereof.
FIG. 2(a) is a sectional view illustrating a conventional printed circuit board 20 embedded with the multilayer chip capacitor 10. Referring to FIG. 2(a), the printed circuit board 20 includes a top multilayer plate 30, a bottom multilayer plate 35, and a core substrate 40 interleaved there between. The multilayer chip capacitor 10 is arranged in a groove formed in the core substrate. External electrodes 11, 13 of the capacitor are coupled to pads 61, 63 through conductive vias 51, 53, respectively.
To manufacture the printed circuit board 20, the capacitor 10 is inserted inside the core substrate 40. Then via holes should be perforated in the top multilayer plate 30 and the bottom multilayer plate 35 by laser to couple substrate wiring to the external electrodes 11, 13 of the capacitor. This laser process increases manufacturing costs of the printed circuit board significantly. Also, smaller size in the multilayer chip capacitor 10 reduces precision of the laser process, rendering it difficult to align via holes adequately. Further, if the board 20 is warped in the process of manufacturing the printed circuit board 20 embedded with the conventional capacitor 10 or handling the printed circuit board 20 manufactured, the capacitor inside the board may sustain mechanical damage. That is, as shown in FIG. 2(b), in case where the board 20 is warped, the thin-film capacitor 10 may undergo cracks 70 easily, thus possibly breaking off the capacitor 10.